The present invention relates to lens control processing performed in an optical apparatus such as a video camera.
It is often the case that an optical apparatus including an optical system of an inner focus type has a function of moving, when moving a lens unit for zooming, a lens unit for focusing based on electronic cam tracking data stored in a memory in advance to correct image plane variation accompanying zooming. This function facilitates zooming while an in-focus state is maintained.
FIG. 8 shows a configuration of a conventional lens system of the inner focus type. In the drawing, reference numeral 901 denotes a fixed front lens, reference numeral 902 denotes a magnification-varying lens (zoom lens) for zooming, reference numeral 903 denotes an aperture stop, reference numeral 904 denotes a fixed lens, and reference numeral 905 denotes a focus lens for focusing and having a correcting function (compensating function) of the image plane variation accompanying zooming. Reference numeral 906 denotes an image pickup surface.
In such a lens system of the inner focus type, even though the lens system has a same focal length, different object distances change an in-focus position of the focus lens 905 for focusing on an object that is a focusing target. When the object distance is changed at each focus length, continuously plotting the in-focus positions of the focus lens 905 at which in-focus object images are formed on the image pickup surface 906 provides plural electronic cam tracks for the respective object distances as shown in FIG. 9.
During zooming performed by moving the zoom lens 902, moving the focus lens 905 so as to trace a cam track selected from the plural electronic cam tracks according to the object distance enables zooming while maintaining an in-focus state.
As shown in FIG. 9, when the zoom lens 902 is moved from a telephoto side to a wide-angle side, the plural cam tracks come closer to each other (converge) from a state where spaces therebetween have a certain width. Hence, it is easy to select one cam track according to the object distance. However, when the zoom lens 902 is moved from the wide-angle side to the telephoto side, it is difficult to accurately select one cam track to be traced by the focus lens from the plural cam tracks having narrow spaces therebetween.
Japanese Patent No. 2901116 discloses an optical apparatus using a TV-AF method which determines a cam track to be traced by a focus lens using an AF evaluation value during zooming. This apparatus compares, during zooming, the AF evaluation values (focus signals) obtained by minutely moving the focus lens in a close direction and an infinite direction with each other to determine a direction in which an in-focus position exists (hereinafter referred to as “in-focus direction”). Then, the apparatus moves a center position of the minute movement of the focus lens by a predetermined amount in the determined in-focus direction to repeat the minute movement, thereby determining one cam track to be traced by the focus lens during zooming.
However, as shown in FIG. 9, the spaces between the cam tracks become wider as the zoom lens approaches the telephoto side. Thus, the apparatus disclosed in Japanese Patent No. 2901116 which determines the in-focus direction by using the AF evaluation values while minutely moving the focus lens has a difficulty of determining the cam track to be traced by the focus lens within a limited zooming time period.
Further, a large movement amount of the focus lens to obtain the AF evaluation values for the plural cam tracks may greatly exceed a depth of focus, which causes defocusing.
Moreover, in the TV-AF method, a cycle of obtaining the AF evaluation value corresponds to a cycle of a vertical synchronizing signal, and hence accuracy of determining the cam track decreases as a zooming speed becomes higher. As a result, a frequency of erroneously determining the cam track increases.
Under these circumstances, Japanese Patent No. 2795439 discloses an optical apparatus capable of maintaining an in-focus state even during high-speed zooming and performing zooming without any dependence on an captured scene or camera work. This apparatus detects a distance (object distance) to a focusing target object, and restricts a movable range of a focus lens for correcting a cam track (that is, for obtaining an AF evaluation value) based on the detected distance.
However, Japanese Patent No. 2795439 does not describe how a cam track to be traced should be corrected within the restricted movable range of the focus lens. Hence, a simple restriction on the movable range of the focus lens causes the following problems in a real image pickup environment.
First, the apparatus disclosed in Japanese Patent No. 2795439 restricts the movable range of the focus lens based on a distance detection result and its detection accuracy. Then, the apparatus determines one cam track to be traced from cam tracks within the movable range. However, as shown in FIG. 2, the movable range becomes wider toward the telephoto side. As a result, in a case where the cam track to be traced by the focus lens is determined by repeating the minute movement of the focus lens in the infinite and close directions around one cam track, zooming may be finished without determining the cam track.
As described above, in the TV-AF method, the cycle of obtaining the AF evaluation values corresponds to the cycle of the vertical synchronizing signal, and hence the number of times of obtaining the AF evaluation values is reduced as the zooming speed increases. As a result, the number of times of moving the center position of the minute movement is reduced. In other words, since the cam track to be traced by the focus lens is determined within the movable range with a small number of times of moving the center position of the movement, erroneous determination of the cam track may often be made, which reduces determination accuracy of the cam track.
Similarly, during a long time exposure such as a so-called slow shutter, even if the zooming speed is not high, the cycle of obtaining the AF evaluation value corresponds to an exposure cycle. As a result, the determination accuracy of the cam track is reduced.
Thus, the optical apparatus disclosed in Japanese Patent No. 2795439 does not determine the cam track to be traced by the focus lens from plural cam tracks for all object distances from an infinite end to a closest end, but narrows down the cam track to be traced within the restricted movable range based on the detection distances. However, when high-speed zooming is performed, a number of and an amount of the center position movement need to be set by taking the number of times of obtaining the AF evaluation value into consideration. Nonetheless, Japanese Patent No. 2795439 discloses no setting method that takes the number of times of obtaining the AF evaluation value into consideration